Generic term for all insulinotropic substances originating in the gastrointestinal tract that are released into the circulation by meals containing glucose. One is glucose-dependent insulinotropic polypeptide, which is released into the circulation from crypt cells in the proximal duodenum and jejunum after meals containing glucose or long-chain fatty acids. Another is proglucagon-derived polypeptide, cleavage product of glucagon, which is further processed into glucagonlike peptide-1 and then to glucagonlike insulinotropic peptide.